EE was operating D8N dozer on stripping dump area, pushing stripping material. While leveling dump area he encountered a slab of rock est. to be 6'x8'x15"". His plan was to remove it from topside of dump. He positioned dozer at or about 45 deg angle to the top of dump site. Dozer traveled over berm & traveled a distance down the slope then slid to left side, came to rest at bottom